Fortress Assault: Guerrilla Warfare

Destruction of enemy supply routes while eliminating the enemy force one group at a time is what's demanded to be able to solo this mission. And as usual for low level TBSs, a Witch is required if you have low gears like the stuff used here.

This map has 2 Summon Gates you will want to destroy to stop more mobs from spawning. Follow the route shown at the map while wiping out groups 1 by 1 as shown on the video. This back route will also avoid having to deal with several mobs and traps on the way to the final Boss.

There are other traps not shown here but you won't have to bother with them if you follow the suggested travel path.

Mission Summary

Since this map has active Summon Gates, start moving out from the start while doing Training upgrades.

Kill mob groups along the way with Witch's AOE, finish with other mercs as needed.

Watch your HP and MP bars and use pots as needed.

Trigger traps with 1 merc, with others located at a safe distance if possible.

Destroy the 1st Summon Gate from a safe distance with Cata and Main (if ranged) to avoid aggroing the mobs near it.

Back to killing 3 mob groups on the way to the 2nd Summon Gate, start encounters with the Witch's AOE and finish off with the rest.

Destroy 2nd Summon Gate.

Move Eastward as shown on the map while clearing mobs and traps.

Kill Boss over the wall with the Witch's AOE, Cata, and Cannon mercs (if any).

Gears shown at the start of this video. Event freebie gears are more than enough to solo this at level 80.